1. Field of the Invention
The present invention relates to a plasma display panel and, in particular, to a plasma display panel in which a driving voltage is lowered, and luminous efficiency is improved.
2. Description of the Related Art
A plasma display panel (PDP) is generally a display device in which vacuum ultraviolet (VUV) rays from plasma generated by gas discharge excite phosphors to emit red (R), green (G), blue (B) visible lights for producing an image.
In a typical alternating current PDP, address electrodes are formed on a rear substrate and extends in one direction. A dielectric layer is formed on the rear substrate to cover the address electrodes. On top of the dielectric layer, barrier ribs positioned between the address electrodes are formed in a stripe pattern, and red(R), green (G) and blue (B) phosphor layers are formed between the barrier ribs
On a first surface of a front substrate facing the rear substrate, formed along one direction crossing the address electrode are display electrodes, each including a pair of a transparent electrode and a bus electrode. A dielectric layer and a MgO protective layer in turn are formed on the entire front substrate covering the display electrodes.
Discharge cells are formed at locations where the address electrodes of the rear substrate cross the display electrodes of the front substrate. The PDP has millions of discharge cells arranged in a matrix pattern.
Such an AC PDP adopts a driving method using memory characteristics to drive such a large number of discharge cells simultaneously. To be specific, a voltage difference over a certain value is necessary to start a discharge between a sustain electrode and a scan electrode, both composing a pair of the display electrodes. A threshold voltage having the certain value is called a firing voltage (Vf).
When an address voltage (Va) is applied between the scan electrode and the address electrode, the discharge starts. The plasma is generated by the discharge in the discharge cell, and the electrons and ions in the plasma move toward the electrodes having the opposite polarity. As a result, the electrical current flows.
Since the dielectric layer is coated on each electrode of the AC PDP, most of the moving space charge is deposited on the dielectric layer with the opposite polarity. Therefore, the net voltage difference across the gap between the scan electrode and the address electrode becomes smaller than the initial address voltage (Va), and that causes the discharge to be weak and disappear eventually.
At this time, the dielectric layer on the scan electrode collects a relatively large amount of the ions, compared to the dielectric layer on the sustain electrode. The accumulated charges on the dielectric layer over the sustain and scan electrodes are called the wall charge (Qw). Also, the voltage across the space between the sustain and scan electrodes is called the wall voltage (Vw).
For the case where a certain voltage (Vs; discharge sustain voltage) is applied between the sustain electrode and the scan electrode successively, the discharge starts in the discharge cell when the sum (Vs+Vw) of the discharge sustain voltage (Vs) and the wall voltage (Vw) exceeds the firing voltage (Vf). The vacuum ultraviolet ray generated at this moment excites the corresponding phosphor layer so that visible lights are emitted, and the transparent front substrate transmits the visible light to show an image.
In the manufacturing process of a PDP, however, a dielectric layer is formed uniform in thickness over the scan and sustain electrodes on the front substrate. While the dielectric layer with a uniform thickness facilitates the manufacturing process, it lowers a discharge efficiency due to a long path in the electric field where the surface discharge occurs between the sustain electrode and the scan electrode.
Therefore, a higher discharge sustain voltage (Vs) is required between the sustain electrode and the scan electrode and leads to a higher discharge voltage for driving the PDP. As a result, the PDP need a large electric power consumption and emits electromagnetic interference resulted from the high voltage driving.
Also, the uniform thickness of the dielectric layer formed between the sustain electrode and the scan electrode reduces the transmittance of the generated visible light through the front substrate and results in a PDP with low luminance and low luminous efficiency.